This invention relates to the production of an organic pigment dispersion by dispersing dried soft-textured organic pigment in a dispersion vehicle.
Pigment dispersion for printing ink use are normally made using a Sigma blade mixer wherein the organic pigment in a presscake form is added stepwise along with the flushing vehicle and/or alkyd to break the water from the presscake. The resulting final concentrated paste is then dried under vacuum to low moisture levels and cutback with additional vehicles and/or solvents to a standard product. This process is known in the trade as a flushing process.
Another method commonly employed in the ink industry is to make a premix of the dry pigment in the vehicle of choice with other components like alkyds, wax, etc. and run this over a 3-roll mill to get the dispersion. The final product is then cut back with vehicles and solvents to a standard product.
A third approach is to make the pigment dispersion using an extruder as outlined in U.S. Pat. No. 4,474,473 feeding a pigment presscake and a hydrophobic liquid organic media into a twin screw co-rotating twin screw extruder.
A fourth method as described in U.S. Pat. No. 5,985,019 is to disperse the pigment granules of a mean size of 0.1 to 50 mm in a twin bladed Z blade kneader or a twin screw extruder with a heatset ink varnish.
The flushing process involves the use of a wet presscake containing about 60-70% water, and the need to eliminate all this water by flushing and drying lengthens the overall processing time. The dispersion obtained in the flushing process is good.
The 3-roll milling process can only be done with lower pigment concentration than the flushing process, but also takes long processing times.
The presscake flushing using the extruder as outlined in U.S. Pat. No. 4,474,473 needs a correct feeding rate of presscake and the vehicle to achieve good dispersion. If the rates are not maintained very accurately, the resulting product can be very weak in tinctorial strength.
The pigment concentrate manufactured as outlined in U.S. Pat. No. 5,985,019 also involves a very accurate feeding of the granules and the ink vehicles in the twin screw extruder to get a good dispersion. The twin screw extruder process also involves the formation of lumps if the feed rates are not controlled properly and results in 3-roll milling the final product for ink making. The Z blade kneader method using the granules and the heatset ink varnish restricts the productivity to the size of the kneader which is normally between 500-1000 gallons. About 4000 to 7000 lbs of pigment concentrate can be made using the 500 or 1000 gallon kneaders.
The present invention provides a process for the preparation of an organic pigment dispersion comprising dispersing a dried soft-textured organic pigment in a dispersion vehicle, wherein said dispersion vehicle comprises an ink varnish and alkyd and said dispersion is carried out in a tank equipped with a sweep and a high speed dispersion blade.
It has now surprisingly been found that organic pigment dispersion can be prepared in a tank equipped with a sweep and a high speed dispersion blade by dispersing a dried soft-textured organic pigment in a dispersion vehicle that contains an ink varnish and alkyd. Preferably, the viscosity of said dispersion vehicle is from about 20 to about 60 poises.
Typically, the pigment dispersion contains from about 35% to about 80% pigment by weight. Also typically, the ink varnish contains an ink resin and petroleum distillate which plays a role in decreasing viscosity.
The dried soft-textured organic pigment can be granulated or dry-powdered. The dried pigment granules can contain up to 5% moisture. Examples of pigments include but are not limited to C.I. Pigment Yellow 12, lithol rubine azo, azomethene, copper phthalocyanine, anthraquinone, nitro, perinone, quinacridone, azo or azomethine metal salt or complex, dipyrolopyrole pigment, and mixtures thereof. Preferably, the pigment is C.I. Pigment Yellow or lithol rubine.
The present invention avoids all the shortcomings of the prior art described above. Preferably, the pigment is added to the dispersion vehicle during dispersion in a step-wise fashion. The step wise addition helps in building up the viscosity slowly and promotes better dispersion. The presence of the flushing vehicle along with the alkyd and the necessary oil wets the pigment surface extremely well to obtain a good dispersion. The other major advantage is that the process can be carried out in large tanks where the batch size can reach as high as 25000 lbs or more.
The present invention produces a bulkable and pumpable pigment dispersion which can be directly used without the need of 3-roll milling of the ink, making it easier to handle than the concentrated conventional flush pastes.